According to the Centers for Disease Control and Prevention (CDC) and the Advisory Committee on Immunization Practices (ACIP), influenza viruses typically circulate widely in the United States annually, from the late fall through early spring. Although most persons who become infected with influenza viruses will recover without complications, influenza can cause serious illness and death, particularly among older adults, very young children, pregnant women, and those with chronic medical conditions. Annual influenza vaccination is the primary means of preventing influenza and its complications.